1. Field of the Invention
The present invention relates to an apparatus and method for driving a light emitting panel using capacitive light emitting elements such as organic electroluminescence elements or the like.
2. Description of the Related Background Art
In recent years, with the trend of increasing the size of display devices, thinner display devices have been required, and a variety of thin display devices have been brought into practical use. An electroluminescence display comprising a plurality of organic electroluminescence elements arranged in a matrix has drawn attention as one of the thin display devices.
The organic electroluminescence element (hereinafter simply called the “EL element” as well) may be electrically represented as an equivalent circuit as illustrated in FIG. 1. As can be seen from the figure, the element can be replaced with a circuit configuration having a capacitive component C and a component E of a diode characteristic coupled in parallel with the capacitive component. Thus, the EL element can be regarded as a capacitive light-emitting element. As the EL element is applied with a direct current light-emission driving voltage across the electrodes, a charge is accumulated in the capacitive element C. Subsequently, when the applied voltage exceeds a barrier voltage or a light emission threshold voltage inherent to the element, a current begins flowing from one electrode (on the anode side of the diode component E) to the organic functional layer which carries the light emitting layer so that light is emitted therefrom at an intensity proportional to the current.
The Voltage V-Current I-Luminance L characteristic of the element is similar to the characteristic of a diode, as illustrated in FIG. 2. Specifically, the current I is extremely small at a light emission threshold voltage Vth or lower, and sharply increases as the voltage increases to the light emission threshold voltage Vth or higher. The current is substantially proportional to the luminance L. Such an element, when applied with a driving voltage exceeding the light emission threshold voltage Vth, exhibits a light emission luminance in proportion to a current corresponding to the applied driving voltage. On the other hand, the light emission luminance remains equal to zero when the driving voltage applied to the element is at the light emission threshold voltage Vth or lower which does not cause the driving current to flow into the light emitting layer.
As a method of driving a display panel using a plurality of EL elements, a simple matrix driving system is known. FIG. 3 illustrates the structure of a driver applied with the simple matrix driving system. In a light emitting panel, n cathode lines (metal electrodes) B1 -Bn are arranged extending in parallel in the horizontal direction, and m anode lines (transparent electrodes) A1-Am are arranged extending in parallel in the vertical direction. At each portion where the cathode lines and the anode lines (a total of n×m locations) intersect, an EL element E1,1-Em,n is formed. The elements E1,1-Em,n which carry pixels are arranged in matrix, at the intersections of the anode lines A1-Am along the vertical direction and the cathode lines B1-Bn along the horizontal direction. The elements E1,1-Em,n have one end connected to an anode line (on the anode line side of the diode component E in the aforementioned equivalent circuit) and the other end connected to a cathode line (on the cathode line side of the diode component E in the aforementioned equivalent circuit). The cathode lines are connected to a cathode line scanning circuit 1, while the anode lines are connected to an anode line drive circuit 2.
The cathode line scanning circuit 1 has scanning switches 51-5n corresponding to the cathode lines B1-Bn for individually determining potentials thereon. Each of the scanning switches 51-5n supplies a corresponding cathode line either with a positive potential VCC (for example, 10 volts) or with a ground potential (0 volt).
The anode line drive circuit 2 has current sources 21-2m (for example, constant current sources) and drive switches 61-6m corresponding to the anode lines A1-Am for individually supplying the EL elements with driving currents. Each of the drive switches 61-6m is adapted to supply an associated anode line with the output of the current source 21-2m or a ground potential. Each of the current sources 21-2m has an amount of supply current which is required to maintain light emitting of the EL elements at desired instantaneous luminance (hereinafter this state is called the “steady light emitting state”). Also, When an EL element is in the steady light emitting state, the aforementioned capacitive component C of the EL element is charged, so that the voltage across both terminals of the element becomes a positive value Ve (hereinafter, this value is called the “light emission regulating voltage”) slightly higher than a light emitting threshold voltage Vth. It should be noted that when voltage sources are used as driving sources, their driving voltages are set to be equal to Ve.
The cathode line scanning circuit 1 and the anode line drive circuit 2 are connected to a light emission control circuit 4.
The light emission control circuit 4 controls the cathode line scanning circuit 1 and the anode line drive circuit 2 in accordance to image data supplied from an image data generating system, not shown, so as to display an image represented by the image data. The light emission control circuit 4 generates a scanning line selection control signal for controlling the cathode line scanning circuit 1 to switch the scanning switch 51-5n such that any of the cathode lines corresponding to a horizontal scanning period of the image data is selected and set at the ground potential, and the remaining cathode lines are applied with the positive potential VCC. The positive potential VCC is applied by regulated voltage sources connected to cathode lines in order to prevent crosstalk light emission from occurring in EL elements connected to intersections of a driven anode line and cathode lines which are not selected for scanning. The positive potential VCC is typically set equal to the light emission regulating voltage Ve (VCC=Ve). As the scanning switches 51-5n are sequentially switched to the ground potential in each horizontal scanning period, a cathode line set at the ground potential functions as a scanning line which enables the EL elements connected thereto to emit light.
The anode line drive circuit 2 conducts a light emission control for the scanning lines as mentioned above. The light emission control circuit 4 generates a drive control signal (driving pulse) in accordance with pixel information. The drive control signal is a signal for instructing which of EL elements connected to associated scanning lines are driven to emit light at which timing and for approximately how long, and supplies the drive control signal to the anode line drive circuit 2. The anode line drive circuit 2, responsive to this drive control signal, individually controls the switching of the drive switches 61-6m to supply driving currents to associated EL elements through the anode lines A1-Am in accordance with the pixel information. Thus, the EL elements supplied with the driving currents are forced to emit light in accordance with the pixel information.
Next, the light emitting operation will be described with reference to an example illustrated in FIGS. 3 and 4. This light emitting operation is taken as an example in which a cathode line B1 is scanned to have EL elements E1,1 and E2,1 emit light, and subsequently, a cathode line B2 is scanned to have EL elements E2,2 and E3,2 emit light. Also, for facilitating the understanding of the explanation, in FIGS. 3 and 4, an EL element which is emitting light is represented by a diode symbol, while an element which is not emitting light is represented by a capacitor symbol.
Referring first to FIG. 3, only a scanning switch 51 is switched to the ground potential equal to zero volt to scan a cathode line B1. The remaining cathode lines B2-Bn are applied with the positive potential VCC through the scanning switches 52-5n. Simultaneously, anode lines A1 and A2 are connected to current sources 21 and 22 through drive switches 61 and 62, respectively. The remaining anode lines A3-Am are switched to the ground potential at zero volt through drive switch 63-6m. Thus, in this event, only the EL elements E1,1 and E2,1 are forward biased so that driving currents flow thereinto from the current sources 21 and 22 as indicated by arrows, causing only the EL elements E1,1 and E2,1 to emit light. In this state, the EL elements E3,2 and Em,n which are not emitting light, indicated by hatching, are charged with polarities as indicated in the drawing.
From the light emitting state illustrated in FIG. 3, only the scanning switch 52 corresponding to the cathode line B2 is now switched to the ground potential at zero volt to scan the cathode line B2 as illustrated in FIG. 4. Simultaneously with this scanning, the current sources 22, 23 are connected to the corresponding anode lines A2, A3 through the drive switches 62, 63, while the remaining anode lines A1, A4-Am are applied with zero volt through the drive switches 61, 64-6m, respectively. Thus, in this event, only the EL elements E2,2, E3,2 are forward biased, so that driving currents flow into the EL elements E2,2, E3,2 from the current sources 22, 23 as indicated by arrows, causing only the EL elements E2,2, E3,2 to emit light.
In the light emitting control as described above, a scanning mode that is a period in which any of the cathode lines B1-Bn is activated is repeated. The scanning mode is performed every one horizontal scanning period (1H) of image data, wherein the scanning switches 51-5n are sequentially switched to the ground potential every horizontal scanning period. The light emission control circuit 4 generates a drive control signal (driving pulse) in accordance with pixel information. The drive control signal instructs which of EL elements connected to associated scanning lines are driven to emit light at which timing and for approximately how long, and is supplied to the anode line drive circuit 2. The anode line drive circuit 2, responsive to the drive control signal, controls the switching of the drive switches 61-6m to supply driving currents to associated EL elements according to the pixel information through the anode lines A1-Am. Thus, the EL elements supplied with the driving currents perform light emitting corresponding to the pixel information.
During a period in which the cathode line B1 is selected and driven at the ground potential, EL elements E3,2-Em,n are applied with the voltage Vcc in the direction opposite to the forward direction to prevent EL elements on non-selected scanning lines from emitting light to cause crosstalk, so that the EL elements E3,2-Em,n are charged.
However, since the charge accumulated in the reverse direction for purposes of preventing the crosstalk light emission is a charge which never contributes to light emission, useless power is consumed.
Also, immediately after the scanning is switched from the cathode line B1 to the cathode line B2, the EL element E3,2, which is one of the charged EL elements, has the anode connected to the current source 23 through the drive switch 63, and the cathode driven to the ground potential through the scanning switch 52, so that the EL element E3,2 should emit light. However, unless the charge accumulated on the EL element E3,2 in the reverse direction has been discharged, the EL element E3,2 is not immediately applied with a voltage exceeding the light emission threshold voltage Vth in the forward direction. Therefore, there is a problem that a delay occurs before the EL element E3,2 actually emits light.